A stratified scavenging engine is used in, for example, a chain saw, a brush cutter and an edge trimmer. In the stratified scavenging engine, mixture consisting of fuel and air, and stratified scavenging air are supplied to a cylinder block through respective suction tubes. Generally, the mixture produced in a carburetor is supplied to the cylinder block through a suction tube for mixture, while the stratified scavenging air is supplied to the cylinder block through a suction tube for air, which does not pass through the carburetor (refer to, for example, the Patent Publication 1).
On the other hand, another kind of stratified scavenging engine in which both of mixture and stratified scavenging air are supplied through a carburetor is known (refer to, for example, Patent Publication 2). In this carburetor of the stratified scavenging engine, one bore is formed, and a butterfly-type choke valve and a butterfly-type throttle valve are disposed in the one bore so that two passages are defined in the one bore when both of the valves are full-opened. Then, the mixture flows through the passage on one side of the full-opened valves (or the passage provided with a fuel injection aperture), while the stratified scavenging air flows through the passage on the other side.
In this kind of the stratified scavenging engine, a suction tube disposed between the cylinder block and the carburetor is a one suction tube into which a passage for mixture and a passage for air are integrated. Concretely, as shown in FIG. 5, an entire inlet cross-sectional profile of the suction tube 102 of the stratified scavenging engine 100 corresponds to a cross-sectional profile of a bore (not shown) of the carburetor (not shown), and the passage for mixture 106 and the passage for air 108 are defined by a common partition 104 in a thin plate form which aligns with the throttle valve in the fully-opened position. In this way, by integrating the passage for mixture 106 and the passage for air 108 into the one suction tube 102, the stratified scavenging engine 100 can be made compact.
Further, the suction tube is made of a rubber or a resin in order to prevent vibration and heat of the engine 100 from being transmitted to the carburetor. In a chain saw, the suction tube is typically made of a rubber.